An image shot, for instance, by a digital camera is not always oriented in a proper direction depending on how the camera is held. To provide efficient image management, therefore, it is desirable that the orientation of the image be corrected as needed to display the image in proper orientation.
An image orientation determination method for automatically determining the horizon to zenith direction of an image is described in Patent Document 1. The determination method described in Patent Document 1 analyzes pixel values of a color image to acquire predefined image feature information, which describes the features of the color image. A determination rule based on the image feature information is then used to determine the horizon to zenith direction of the color image.
A method of recognizing the top and bottom of an image is described in Patent Document 2. The method described in Patent Document 2 sets a substantially U-shaped area, which is formed along three of four sides of an image outline, as a determination area for identifying the top and bottom of a designated image, obtains the maximum and minimum values of pixels within the determination area, and identifies the top and bottom of the image in accordance with the difference between the maximum and minimum values.
A face image orientation determination method for accurately determining the orientation of an image including a human face as a subject is described in Patent Document 3. The determination method described in Patent Document 3 acquires a face image including a human face, and detects eyes and a mouth from the acquired face image. This determination method determines the orientation of the face image in accordance with the positional relationship between the detected eyes and mouth in the acquired face image.
An image upper side determination method for determining how a camera is held for shooting is described in Patent Document 4. The determination method described in Patent Document 4 compares total brightness information values of four corners of an image, recognizes the brightest side as the top end of the image, that is, as the upper side of the image when it is viewed, and eventually determines how the camera is held.
A determination method for determining the orientation of an image in accordance with its composition is described in Non-Patent Document 1. The method described in Non-Patent Document 1 extracts low-level features of the image, and determines the orientation of the image by allowing the low-level features of a sample image to be machine-learned.